Effect of Agricultural Organic Inputs on Nanoplastics Transport in Saturated Goethite-Coated Porous Media: Particle Size Selectivity and Role of Dissolved Organic Matter

The transport of nanoplastics (NPs) through porous media is influenced by dissolved organic matter (DOM) released from agricultural organic inputs. Here, cotransport of NPs with three types of DOM (biocharDOM (BCDOM), wheat strawDOM (WSDOM), and swine manureDOM (SMDOM)) was investigated in saturated goethite (GT)-coated sand columns. The results showed that codeposition of 50 nm NPs (50NPs) with DOM occurred due to the formation of a GT–DOM–50NPs complex, while DOM loaded on GT-coated sand and 400 nm NPs (400NPs) aided 400NPs transport due to electrostatic repulsion. According to the quantum chemical calculation, humic acid and cellulose played a significant role in 50NPs retardation. Owing to its high concentration, moderate humification index (HIX), and cellulose content, SMDOM exhibited the highest retardation of 50NPs transport and promoting effect on 400NPs transport. Owing to a high HIX, the effect of BCDOM on the mobility of 400NPs was higher than that of WSDOM. However, high cellulose content in WSDOM caused it to exhibit a 50NPs retardation ability that was similar to that of BCDOM. Our results highlight the particle size selectivity and significant influence of DOM type on the transport of NPs and elucidate their quantum and colloidal chemical-interface mechanisms in a typical agricultural environment.

S9. Fitted parameters of nanoplastics transport in the GT coated sand columns S10. DLVO interaction energy between nanoplastics and (GT coated) sand S11. The contents of starch, hemicellulose, cellulose, and lignin in the agricultural organic inputs S12. FTIR differential spectra analysis S13. Transport of different DOM S14. Fitted parameters of DOM transport in the GT coated sand columns S15. Fitted parameters of nanoplastics co-transport with DOM in the GT coated sand columns S16. Stability of DOM and its influence on the stability of nanoplastics S17. DLVO interaction energy between nanoplastics and sand before and after co-transport with DOM in the GT coated sand columns S19. Deposition of nanoplastics S20. XPS results of nanoplastics and different DOM co-deposited on GT coated sand S21. Result parameters of quantum chemical computation 1 S1. Goethite preparation Goethite was prepared with adding 2.5 M NaOH at a speed of 10 ml min -1 to 5 L 0.5 M Fe(NO 3 ) 3 solution. Keep on mixing the suspension during the addition. And put a pH-electrode in the above suspension to monitor the pH and stop adding NaOH when pH = 12. Then put the suspension in an oven at 60 C for 4 days to let Fe(OH) 3  According to the soluble organic components detected in the fluorescence spectral distribution, excitation/emission (E x /E m ) wavelength regions can be considered as humic acid-like areas to demonstrate the humic characteristics of these components 1 .
The humification index (HIX) was calculated as the ratio of the peak integrated area of emission wavelengths ranging from 300 to 345 nm to that of emission wavelengths ranging from 435 to 480 nm, under a 255-nm excitation wavelength 2 .   Figure S2. The calibration curves of 50NPs (a), 400NPs (b), and NPs-DOM suspension (c-t) concentrations between absorbency and standards at pH 6.0.

S5. Nanoparticles transport models
The convection diffusion equation (CDE) with two kinetic retention sites was employed to describe the nanoparticle transport and retention in the column experiments as equation (1)  (1) is the column dry bulk density; x (cm) is the spatial coordinate; ν (cm·min -1 ) is the Darcy's velocity; and S 1 (g·g -1 ) and S 2 (g·g -1 ) are nanoparticle concentrations deposited in Site1 and Site2, respectively.
The Site1, first kinetic site, on which the retention of the nanoparticle is assumed to be reversible, whereas Site2, the second kinetic site, on which the retention is assumed to be irreversible, as described by the depth-dependent retention. S 1 on Site1 and S 2 on Site2 are given in equations (2) and (3), respectively.
k 1a (min -1 ) and k 2a (min -1 ) are first-order retention coefficients on Site1 and Site2, respectively; k 1d (min -1 ) is the first-order detachment coefficient; ψ t (dimensionless) is the nanoparticle attachment function to account for the depth-dependent behavior of particle attachment expressed by equations (4): -β d c is the median diameter of the sand grains (cm); x 0 is the coordinate of the location where the straining process starts; and β (dimensionless) is an empirical variable that controls the shape of the retention profile, using an optimal value of 0.432 for different sized spherical nanoparticle and sand grains in which significant depthdependency (hyperexponential retention profiles) occurred 4 . Three parameters, including k 1a , k 2a , and k 1d , were fitted.

S6. DLVO theory
The representative Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used to qualitatively understand the NPs transport and retention in water-saturated sands columns through calculating the total particle-sand interaction energy as the sum of Lifshitz-van der Waals (LW) and electrical double layer (EDL) interactions 5, 6 . Ionic strength stays constant at 0.1 mM NaCl. The equation of the LW interaction energy (E LW ) is given as follows 7, 8 : d p is the diameter of nanoparticle; h is the separation distance between the nanoparticle and sand surface; λ is the characteristic wavelength of interaction and was defined as 100 nm; A 132 is the Hamaker constant of particle-water-sand, which can be expressed by equation (6): A 11 is the Hamaker constant for NPs (6.60 × 10 -20 J) 9 ; A 22 is the Hamaker constant for quartz sand (8.86 × 10 -20 J) 10 ; A 33 is the Hamaker constant for water (3.7 × 10 -20 J) 9 .
The equation of EDL interaction energy (E EDL ) is given as follows 11, 12 : ε 0 is the dielectric permittivity of vacuum (8.854×10 -12 F·m -1 ); ε r is the relative dielectric permittivity of water (78.5); ψ p and ψ c are the zeta potentials of NPs and GTcoated sand, respectively; κ (m -1 ) is the Debye-Hüchel parameter, which is expressed by equation (8);

S7. Typical DOM selection reason and their molecular formula
A part of the polysaccharides in WS DOM and SM DOM may be directly or indirectly derived from the cellulose (CL) in plant cell walls; thus, CL was selected to represent polysaccharides. Amylose (AM) was also selected as a common polysaccharide. Both

S8. Calculation of binding energy
The equation of binding energy between different species NPs and DOM is given as follows: Where E complex represents the energy of a complex composed of two molecules, and E Fragment1 and E Fragment2 represent the energy of a single molecule corresponding to different systems.

S11. The contents of starch, hemicellulose, cellulose, and lignin in the agricultural organic inputs
The contents of starch, hemicellulose, cellulose, and lignin in BC, SW, and SM were determined using an enzymatic method 15,16 .

S12. FTIR characteristics of nanoplastics and FTIR differential spectra analysis
The series absorption peaks at 3085, 3062, and 3025 cm -

S13. Transport of different DOM
For individual DOM transport, particularly in 2% GT-coated 70-μm sand, the high content of GT and fine sand might cause DOM retention in the column (Fig. S6).
Negatively charged DOM was readily adsorbed on GT during transport, forming a ligand exchange between the carboxyl/hydroxyl functional groups of DOM and the GT surface 17 . The retention of DOM significantly changed the properties of the GT-coated sand. The retention rate was the highest (average: 43.0%) in BC DOM because of its low concentration (Table S6); however, the amount retained in the column was low. The retention rate of WS DOM (average: 36.2%) was higher than that of SM DOM (average: 20.9%) (Table S6). WS DOM was readily deposited in the 2% GT-coated 70-μm sand column (71%) (Fig. S6 and Table S6) because the protein-like substance in DOM promoted the formation of bridged complexes with GT and organic molecules 18 , and the small pore structure facilitated this process.

S16. Stability of DOM and its influence on the stability of nanoplastics
Settling      Figure S11. X-ray photoelectron spectroscopy of 50NPs co-transport with different DOM deposited on GT-coated sand. Data was identified by X-ray photoelectron spectroscopy with an Al Kα X-ray source (1486.6 eV). Survey spectra were recorded from 1200 ~ 0 eV for each sample in a vacuum of 8 ×10 -10 Pa. All peaks were calibrated using the C1s peak at 284.8 eV.
The data was processed using the XPSPEAK 4.1.